RELATED ART STATEMENT
A high intensity discharge lamp filled with an ionizable material containing a metal halide compound is disclosed in U.S. Pat. No. 4,409,517. As a metal constituting electrodes for the high intensity discharge lamps of this kind, niobium, tantalum or the like have been used because the metal is required to have a coefficient of thermal expansion near that of a ceramic tube, particularly, an alumina tube. However, niobium and tantalum have not necessarily resistance large enough to withstand the halogen. For this reason, JP-A-6 196,131 disclosed use of a ceramic tube designed as shown in FIG. 9.
This ceramic tube includes a tubular body 11 in which a discharging space 4 is formed, a pair of annular members 12 inserted into opposite open end portions of the tubular body 11, and slender, cylindrical electrode-inserting members 13 fixedly inserted into central openings of the annular member 12, respectively. Pressure is exerted between the inner peripheral face 11a of the tubular body 11 and each of the annular member 12 and between the annular member 12 and the electrode-inserting member 13 due to firing shrinkage. Thereby, the members 11, 12 and 13 are integrally sintered through the firing shrinkage so that gas tightness may be maintained among them. An electrode member not shown is inserted through a space 2 of each of the electrode-inserting members 13.